The present invention relates to a glass substrate in the form of a disk for recording information, and more particularly, to a glass substrate in the form of a disk, for use in a fixed type magnetic disk recorder and in a photomagnetic disk apparatus, which is superior in chemical stability and thermal stability at a high temperature and in flatness. The invention also relates to a disk for recording information and a disk apparatus for recording information using the glass substrate.
Magnetic disks and photomagnetic disks are used at the present time as media for recording information in large scale computers and personal computers. Currently, substantial advances have been made in increasing the recording density and decreasing the size and weight of the media for recording information. In the field of the magnetic disk apparatus, magnetic disk substrates of small size, such as disks having diameters of 2.5 inches, 1.8 inches and 1.3 inches, have been developed. Not only the diameter, but the thickness of the disk substrate as well has been decreased. Accordingly, a substrate having a larger mechanical strength then ever is required. Furthermore, the levitating amount of the disk on the head has been decreased, and improvement in the flatness of the disk, which is capable of controlling the levitating amount, is required.
Conventionally, an aluminum alloy has been used as the material of the substrate for a magnetic disk. However, an aluminum alloy has the problem that it is deformed readily and exhibits an undesirable flatness at the disk surface after polishing on account of insufficient hardness. Furthermore, an aluminum alloy has another problem in that the layer of the magnetic recording medium is readily peeled off when the magnetic disk is contacted mechanically by the head. Therefore, a glass substrate having a smaller deformation, a more preferable flatness and a larger mechanical strength than the aluminum substrate has been developed. The glass substrate can be used also as the substrate for a photomagnetic disk, wherein the substrate itself is required to be transparent.
In order to use a glass substrate as the media to record information, chemically strengthened glass, having a mechanical strength which is increased by replacing alkali elements at the surface of the substrate with other alkali elements, and crystallized glass, wherein fine crystallized particles are deposited in an amorphous glass matrix, have been developed. However, the chemically strengthened glass has a problem in that the alkali ions tend to corrode the magnetic film and deteriorate the adhesive strength of the film during a heating step and a cleaning step in the magnetic film forming process, because of the presence of the alkali substituted layer at the surface of the film. Accordingly, JP-A-7-223845 (1995) has proposed a chemically strengthened glass, which was superior in chemical resistance, prepared by a further chemical treatment after the chemical strengthening.
On the other hand, crystallized glass has a problem in that the irregularity of the surface of the glass substrate after polishing was very large, because of the difference in abrasion resistance between the generated crystalline fine particles and the amorphous glass, so that a desirable flatness of the surface could not be obtained. Therefore, decreasing the levitating amount of the head was difficult for responding. In accordance with JP-A-7-300340 (1995), the flatness was improved by decreasing the particle size of the crystallized fine particles to the range of 5-100 nm.
However, the chemical strengthening method disclosed in JP-A-7-223845 (1995) has a problem in that making the characteristics of the substrate homogeneous after the chemical strengthening process and the polishing process in the chemical strengthening treatment is still difficult. Furthermore, providing an effective intermediate film to prevent a diffusion of alkaline ions between the magnetic film and the glass substrate, which is necessary to prevent the magnetic film from being corroded by the alkaline ions, has not been solved completely yet. In accordance with the crystallizing method disclosed in JP-A-7-300340 (1995), the obtained glass substrate has a low transmittance because the glass is colored to a milky color, and accordingly, the glass substrate is difficult to use as a substrate for optical information recording. Furthermore, the crystallizing method has another problem in that making the characteristics of the substrate homogeneous is difficult, because a fine precise crystallizing process is required.
In either case using the above methods, decreasing the production cost is difficult, because a post-treating process, such as a chemical strengthening process or a crystallizing process, is required.